Carpet!
Just covered my subbie box with a great match.
Went to a rubber/ vinyl / carpet flooring place and they sell you charcoal coloured thick carpet (same as speaker box) much cheaper than from stero shops (I checked them out and they were pricey ++).
It's a perfect match - when you look in the back you can't really tell theres a bin in there at all, my bin sits under the parcel tray behind the seats, firing into them.
good luck
rich